1. Field of the Invention
The present invention relates to an improved apparatus and process for providing a thermal radiation heat sink in a fuel bundle having a large water rod for use in a boiling water nuclear reactor. The apparatus and process provide the dual functions of neutron moderation during normal operating conditions and radiation sink during a loss of coolant accident.
2. Description of Related Art
The present invention is applicable to boiling water reactors of the type wherein an overhead core cooling spray (CCS) is used during loss of coolant accidents (LOCAs). Specifically, the invention relates to fuel bundle assemblies in which a relatively large diameter water rod is disposed within the array of fuel rods to provide increased neutron moderation. For the purposes of this invention, "large" is used to describe a tube having a diameter at least twice the diameter of the fuel rods. The large water rod is usually centrally located and under normal operating conditions is filled with water as a neutron moderator to provide an even power output distribution across the fuel bundle.
In a LOCA situation, the water drains out of the fuel assemblies and the large water rod. The control rods are fully inserted to stop the fission, but residual energy due to decay of fission products continues to be produced for a period of time. The emergency CCS is activated and water sprays across the top of the fuel assemblies from a number of directions The upper tie plate, which ties together the fuel assemblies, partially obscures the fuel rods from the water. Due to the depressurization and high temperatures associated with a LOCA, much of the spray flashes to steam. In order to dissipate the residual heat effectively, it is necessary to provide heat sinks for the fuel rods wherever possible.
The large water rod is orificed at its lower end and discharges water under normal operating conditions into the fuel bundle through holes just beyond the active fuel length.